When photons impinge on a target, the photons interact with the material of the target in various ways. The mode of interaction between the photons and the material of the target depends on the energy of the photons and the atomic number of the atoms of the material of the object. For example, the graph in FIG. 1 illustrates the dominant photon interaction type as a function of Z and photon energy as originally published in Oldenberg, Modern Physics For Engineers, New York, McGraw-Hill (1966).
The angular distribution of Compton scattering shows a clear forward bias (i.e. away from the detector) with increased energy as seen in FIG. 2, which was created using the Wolfram Demonstration Project website (20 Jul. 2010), S. M. Blinder. Klein-Nishina Formula for Photon-Electron Scattering. This implies that, despite the increased photon range from higher energy photons and increased probability of Compton scattering in comparison to photoelectric interactions, the net result is that each scattering interaction is less efficient, creating a balance between higher ranger and thus deeper depth penetration but lower efficiency per interaction. The probability is expressed in terms of mass attenuation coefficient (μt), with the probability of not interacting and thus not being scattered exponentially decreasing with increased μt and material thickness. Each interaction type has its own μ, such as μpe for photoelectric effect and μcs for Compton scattering.
Higher energy photons, for the same material, would be more likely to result in scattering events, although each scattering event is less efficient in producing backscatter photons. Similarly, for monoenergetic photons at lower energies, Compton scattering is dominant in low Z materials until it is overtaken by increasing μpe relative to μcs with increased Z, as seen in FIG. 1. For an ideal single material, μcs must be very high in comparison to μpe and the actual value for μ must be low enough such that a large proportion of photons do not interact prior to the critical depth. These considerations make it clear that high Z materials are not ideal materials to image using either transmission x-ray imaging or Radiography by Selective Detection (RSD). Low Z materials not only have significantly more Compton scattering at low energies, but a greater proportion of scattering events result in backscatter due to the low energies necessary. For these reasons, imaging of high-Z materials with an X-ray radiation has so far been considered impossible.